The dramatic increase in the price of gold over the past several years has given rise to new methods and equipment in the electroplating field and attempts to use substitute metals such as palladium through selective plating. Specialized plating equipment such as described, for example, in U.S. Pat. Nos. 3,723,283 and 3,819,502, and others of a similar nature, utilize chemical or mechanical masks which limit the metal deposits to the specific required area while leaving other areas free of the metal. The use of equipment of this type makes possible a reduction in the amount of metal deposited by a factor of 10 or above compared with equipment and methods utilizing overall plating techniques and results in a like reduction in the cost of the electroplated item produced.
The use of such plating process requires very high speed plating and high speed requires high current densities preferably above 100 ASF. A discussion of some of the problems and advantages of high speed plating is found in U.S. Pat. No. 3,879,269.
Attempts have been made to plate palladium from high speed plating equipment with various baths. However, the deposits are either burned and matte gray or they are bright to semi-bright and highly stressed and exhibit surface microcracks which are visible only under the microscope at high power. These cracks can be visible in the deposit right out of the plating bath or they become visible later or after the deposit has been permitted to stand at room temperature for about a day or so. Much has been written about these cracks and it is generally attributed to the codeposition of hydrogen with palladium, and after the hydrogen is emitted from the deposit cracks will appear. The industry desires to have palladium deposits that are crack-free at usable current densities in high speed plating. The current densities required for high speed plating in today's technology range from about 100 to 2,000 ASF and higher. High speed plating equipment employs the jet plating principle where the solution is sprayed out onto the surface being plated with a jet stream, to provide very vigorous agitation. It is also possible to supply vigorous agitation without the jet stream but by moving the solution very rapidly past the part being plated through the use of a pump and/or by moving the parts rapidly through the solution.
Prior art palladium plating solutions are exemplified by U.S. Pat. Nos. 3,972,787 and 4,092,225, U.S. published patent application No. B450,499 and Russian Pat. No. 354,010.
U.S. Pat. No. 3,972,787 discloses a number of palladium plating baths. The first four specific baths are disclosed as having a current density range from 0.1 to 40 ASF except for Example 5 which was 0.1 to 50 ASF. All of these baths are unsuitable for high speed plating. The patent also discloses that at 20 g/l palladium it is possible to plate at 500 to 1,000 ASF at 50.degree. C., but without the brighteners the panel is dull gray above 4 ASF.
The Russian patent discloses palladium chloride, ammonia ions and ammonia phosphate at a pH of 6.1 and 6.7. The usable current density range of these solutions is 6 to 8 ASF, and they cannot be used in high speed plating.
The published application uses palladium ammonium chloride and free ammonia at a pH of at least 8.8. These baths contain no phosphate or carbonate anions.
U.S. Pat. No. 4,092,225 adds palladium as Pd(NH.sub.3).sub.2 (NO.sub.2).sub.2 and tetrapotassium pyrophosphate to form a bath at a pH of 8.5 to 11 and discloses that the solutions are suitable for rack plating at 2-50 ASF.